(a) Field of the Invention
The invention relates to a teletext decoder, particularly to a teletext decoder having an automatic correction function.
(b) Description of the Related Art
According to ETS 300 706 enhanced teletext specification from ETSI, a teletext message can comprise various types of packets. FIG. 1 shows a schematic diagram illustrating the structure of a conventional packet. As shown in FIG. 1, the packet 900 comprises a clock run-in 911, a code area 910, and a data area 914. The code area 910 further comprises a teletext framing code 912 and other codes 913. Different kinds of packets 900 may contain different other codes 913 and a data area 914. The other codes 913 can be at least one of the following: magazine and packet number, page, sub-code, control bits, and designation code. The data area 914 can be characters, control codes, or information.
Generally, the packet 900 has 45 bytes, that is, there are 45 8-bit data, wherein the clock run-in 911 has 2 bytes and the teletext framing code 912 has one byte. As a television receives the waveform of a teletext signal, the average of the clock run-in 911 in the packet 900 is calculated as a slicer level for acquiring the digital data in the teletext signal. The television can use the teletext framing code 912 to distinguish whether or not the data from the data area 914 of the packet 900 is a teletext.
Various coding methods, such as odd parity check and Hamming Encode method, can be used to encode the packet 900 in order to ensure data accuracy. The code area 910 comprises less data quantity but is more important. Thus, generally 4 data bits and 4 Hamming protection bits are used to encode (Hamming 8/4 Encode). The data quantity of the data area 914 is larger and usually 7 data bits and one odd parity check bit are used to encode in order to increase the television performance and the data transmission quantity of the packet 900. The odd parity check method does not have any function of correcting an error occurrence bit. When there is any error in the 8-bit data encoded by the odd parity check method, since the odd parity check method does not have any function for correcting error occurrence bit and TV broadcast transmits uni-directionally from the transmitting end, the receiving end can only discard the data with error. In the prior art, when a page or a line with error is detected, the old data is used without update to let user see the screen without any incorrect text. For example, U.S. Pat. No. 4,614,971 disclosed a system for eliminating error in teletext signals. The system provides with a noise detector to determine whether or not the data in the packet temporarily stored in the memory is interfered by noise. When the noise detector detects a certain level of noise, the packet will be discarded and thus will not be input to the teletext decoder so that the probability of displaying the data with error in the packet on the screen is decreased.